This invention pertains to the art of imaging devices and systems, and more particularly digital color imaging devices. The invention is especially applicable as a color calibration technique for a scanning device and a media item being scanned by the device, without having to refer to a dedicated target representative of the media. By media, it is meant the combination of materials used in producing the image, such as in photography, the dyes being used, or in xerography, the different toners.
In particular, the invention comprises a method for target-less color calibration of a color scanner using one model for the scanner's operation and another model for the scanned media. The scanner model computes the scanner signals produced when scanning a given input sample by using the spectrum of the input sample and the predetermined spectral sensitivities for the scanner. The spectral model for the scanned media is derived from representative spectrophotometric measurements (usually from the image/images being scanned). In one embodiment of the invention, a calibration is obtained for a particular scanned color sample, by means of a globally convergent algorithm that uses the scanner signals for the sample along with the two models to obtain the spectral reflectance or transmittance of the scanned color sample. In another embodiment, the globally convergent algorithm is used to generate a look-up table (LUT) which relates scanner signals to input spectrum or to device independent color values obtainable from the spectrum, e.g. conversion of the scanned R,G,B into L*,a*,b*. In yet another embodiment, the LUT is generated by interpolation/extrapolation of correspondences between scanner signals and input spectrum (or device independent color values) obtained by using the scanner model to obtain scanner signals for a number of different candidate spectra generated using the media model. It will be appreciated to those skilled in the art that the invention could be readily adapted for use in other environments as, for example, where similar calibration objectives are required between devices normally requiring a dedicated target for successful implementation of the calibration, e.g. digital color cameras.
Consistent measurement, storage, electronic imaging and printing of a color for production workflow--from scanner, to screen, to proofer, to offset press--is a common problem for which a variety of color management systems have been created. Different devices all display or detect color in a different way, and more importantly, different from the way the human eye sees color. A common illustration of this inconsistency can be observed in any consumer electronics store where hardly any two on a wall of televisions showing the same program appear identical. With particular regard to document imaging systems, the way a scanning device sees color is different from the way the human eye will see color and in order to obtain a consistency between the two, there must be a conversion between the measured image signal generated by the scanner to the printed or displayed representation thereof so that it will appear consistent to the human eye. Thus, the conversion of device-dependent color, i.e., color that is sensed by a scanner or other color image capture apparatus, to device independent color, i.e., color that is perceived by the human eye and can be measured by a calorimeter, comprises generating a look-up table (LUT) that references differences between a specific device's method of representing or detecting a set of colors, and an absolute, independent model of color.
Representing the physical properties of how a particular image is displayed, printed or scanned is called modeling. In the context of the current invention, a spectral response model for a color scanner relates the reflectance/transmittance spectrum of a given color sample to the signals produced by the scanner when the given sample is scanned; and a model for a given medium represents the set of reflectance/transmittance spectra that can be realized in the given medium.
For accurate calibration of a color scanner, there must be some predetermined knowledge of the medium being scanned. Prior art systems calibrate by taking a target, an industry standard medium having several hundred color and tonal values, and measuring these values in a measuring stage. The target is scanned in by the scanner and a mapping is performed to give a conversion of the signal detected by the scanner for a particular location on the target to the corresponding measured color space values. A particular transformation, or LUT, that results will provide an accurate calibration of the scanner.
A particular problem addressed by the subject invention is the difficulty in maintaining an accurate calibration of different devices throughout a production workflow when a media target representative of the scanned samples is not available.
The present invention contemplates a new and improved target-less calibration technique which overcomes this problem and others to provide a spectral-based calibration of a color scanner based upon a model of the medium materials being scanned and the predetermined spectral response of the scanner. Given the scanner responses to a color sample on a given medium, the spectral reflectance/transmittance of the given color sample must lie within a) the set of spectra producible on the medium and b) the set of spectra that can result in the given scanner signals. A method of projections onto convex sets is therefore used in the preferred embodiment of the invention to identify a reflectance/transmittance spectrum that lies within both these sets, which serves as an estimate of the spectral reflectance/transmittance of the given sample. In an alternate embodiment of the invention, the media model is used to generate a set of candidate spectra for which scanner signals are generated by using the scanner model. Interpolation and/or extrapolation is then used to obtain the look-up table (LUT) from scanner signals to media spectrum (or alternately a device independent color space derived from the spectrum).